Multiprotocol Label Switching (MPLS), an IETF initiative, combines or integrates Layer 2 information about network links (bandwidth, latency, utilization) with Layer 3 information (e.g. Internet Protocol information), to simplify and improve IP-packet exchange. MPLS can be implemented for example within a particular system or network.
Via MPLS, IP (Internet Protocol) traffic can be steered on a variety of paths instead of one single path, for example a single path discovered by an interior gateway protocol such as Border Gateway Protocol (BGP). Thus MPLS can provide network operators with flexibility to route traffic around local malfunctions or problems such as congestion, bottlenecks, link failures, and can be used to enable or guarantee particular class or level of service.
IP packets include a header field containing a precise address to which the packet is to be routed. MPLS generates a short fixed-length label that acts as a shorthand representation of an IP packet's header. This is analogous to a U.S. mail ZIP code that represents a region encompassing a city (or portion thereof), street and street number, and which is used to make forwarding decisions about the mail piece (or in the case of MPLS, the IP packet). Packets are forwarded along a Label Switched Path (LSP) where each Label Switch Router (LSR) makes forwarding decisions based solely on the contents of the packet's MPLS label. At each hop, the LSR strips off the existing label from the MPLS header and applies a new label which tells the next hop how to forward the packet. In contrast to MPLS, traditional routing methods through networks cause the precise address information to be evaluated at every router in a packet's path through the network.
MPLS switches and routers or Label Switch Routers (LSRs) evaluate packets and then affix labels to the packets based for example on packet destination. The LSRs assign each packet a label that corresponds to a particular path through the network. Thus all packets assigned the same label, will travel the same path, termed a Label Switched Path (LSP). Labels refer to paths, not endpoints. Thus, packets destined for the same endpoint (e.g., bearing the same IP address) can arrive via different LSPs.
Specifically, the first MPLS device that an IP packet encounters when entering a network, for example a Label Edge Router (LER), can encapsulate or mark the IP packet with a label. The LER analyzes contents of the packet's IP header and then selects an appropriate label to encapsulate the packet. In selecting the label, the MPLS edge router can consider other factors besides the destination address carried in the IP header, for example, type-of-service parameters, and/or other criteria such as Virtual Private Network membership. Subsequent nodes within the network then use the MPLS label (not the IP header) to make forwarding decision for the packet. When MPLS labeled packets leave the network, an edge router removes the labels.
LSPs are somewhat similar to circuit-switched paths in ATM or Frame Relay networks, except that they do not depend on a particular Layer 2 technology. An LSP can be established that crosses multiple Layer 2 transports such as ATM (Asynchronous Transfer Mode), Frame Relay or Ethernet.
MPLS can benefit IP-based networks, for example by a) providing an ability to set the path that the traffic will take through the network, b) providing a mechanism to implement IP based Virtual Private Networks (VPNs) without need for encryption or end-user applications, and c) eliminating multiple layers. Using MPLS, carriers can transfer functions of the ATM control plane to Layer 3, thereby simplifying network management and reducing network complexity. MPLS paths can be based on IGP (Interior Gateway Protocol) best paths, and MPLS VPN traffic can use IGP best effort paths.